Active matrix devices

In the sections on smectic liquid crystals, first the alignment and molecular orientation of surface stabilized ferroelectric liquid crystals (SSFLCs) are treated in detail. Next, the alignment technologies needed for the occurrence of bistability are detailed. Furthermore, liquid crystalline devices made of AFLC materials and the applications of FLC and AFLC materials to active matrix devices are discussed. 

Application of FLC/AFLC Materials to Active matrix Devices... 

High performance displays, such as color displays on mobile phones and computer screens, require high image refresh rate in combination with high display brightness. The patterned electrode structures in these active matrix devices contain electronic switches in the display area. High performance active matrix devices are silicon-based. 

Fluoroaromatic polar LC-materials (Table 6) with low to moderate Ae show a better ratio of /j to clearing point. Owing to their high resistivity even at higher temperature these materials are especially useful in active matrix devices (TFT). 

An active matrix of two-color a-Si H photodetectors has been reported [645], where a n-i-p-i-n switching device is stacked on a two-color p-i-n-i-p structure. 

J Wang and G Yu, Performance Simulation of Active-Matrix OLED Displays, Photonics Asia 2004 Light-Emitting Diode Materials and Devices, Beijing, China, 2004, pp. 32-44. 

Other device architectures include inverted OLEDs. Here the cathode is in intimate contact with the substrate. The organic layers are then deposited onto the cathode in reverse order, i.e., starting with the electron transport material and ending with the HIL. The device is completed with an anode contact. In this case, as above, one of the electrodes is transparent, and light exits from the device through that contact. For example, Bulovic et al. [38], fabricated a device in which Mg/Ag was the bottom contact and ITO the top electrode. The advantage of this type of architecture is that it allows for easier integration with n-type TFTs (see Section 7.5 for a discussion of active-matrix drive OLED displays). 

Many LCDs are based on active-matrix addressing, in which an active device circuit containing one or more TFTs is connected to each pixel. The TFT circuit at each pixel effectively acts as an individual electrical switch that provides the means to store display information on a storage capacitor for the entire frame time, such that the pixel can remain emitting during this entire time rather than for a small fraction of time, as is the case in passive addressing. 

Y. He, R. Hattori, and J. Kanicki, Improved a-Si H TFT pixel electrode circuits for active-matrix organic light-emitting displays, IEEE Trans. Electron Devices, 48, 1322-1325, 2001. 

FIGURE 10.7 Power consumption simulation for a 2.2-in. full-color OLED display using Universal Display s phosphorescent OLEDs, small-molecule fluorescent devices, and polymer OLEDs along with a comparison of the power consumed by an active-matrix liquid crystal display backlight. R G B= 3 6 1, 50% polarizer efficiency, and 30% of pixels lit. (From Mahon, J.K., Adv. Imaging, June, 28, 2003. With permission.)... 

Detailed performance analysis of the OVPD device again revealed comparable or even slightly better device performance of the OVPD processed device compared with the VTE reference. Figure 9.13 shows the luminous efficiency of the PhOLEDs processed by OVPD and VTE. The OVPD-PhOLED reached 25 cd A-1 and the VTE device approximately 24 cd A-1, equivalent to a maximum external quantum efficiency of 7.0 + 0.1%, which is desirable for active matrix applications. Besides these comparable luminous efficiencies, the spectra differ slightly in their shape... 

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